The present invention relates in general to an agile beam steering device and a method of fabricating the same, and more particular, to a diffraction-based beam-steering device made of electro-optic (EO) material.
Liquid-crystal techniques have been applied to the development of agile, dynamic-grating-based, that is, diffraction-based, beam-steering elements in recent years. Currently, the performance of a single element that uses the liquid-crystal techniques is limited to a steering angle of about ±3o with 50% diffraction efficiency and 1 msec frame-refresh rates.
As understood, the limitation of steering angle for liquid-crystal techniques is caused by the fly-back problem that reduces the diffraction efficiency. For example, Serati et al. has disclosed a device that can provide a theoretical steering angle as large as about ±17o. However, due to the diffraction inefficiency in real practice, the steering angle provided by such device is only about ±3o.
Recently, some researchers modeled the electrostatic fields associated with both liquid crystals and electro-optic crystals such as lithium niobate (LiNbO3) and lanthanum-modified lead zirconate titanate (PLZT) in a longitudinal-modulation configuration. The fly-back problem was found to be the result of lack of “crispness” in the spatial phases resets. That is, because of field fringing, the index of refraction gradient is not sharp enough at a given phase reset to facilitate a high-diffraction-efficiency blazed grating. The excessive field fringing is due to the large aspect ratio (AR) of the modulator thickness to electrode thickness, which is about 10:1 in the typical liquid-crystal design. It was also found that if the aspect ratio can be collapsed to about 1:1, the field-fringing problem could be mitigated.
However, in the liquid-crystal based design, it appears that the aspect ratio is set to be 10:1 due to fabrication limitation. Designs based on electro-optic crystals may facilitate a lower aspect ratio compared to liquid-crystal based design. But it has been a challenge historically in applying electro-optic crystals to agile beam steering due to insufficient electro-optic coefficients.
In fact there has demonstrated the use of electro-optic crystals in a dynamic phase grating. For example, researchers in the telecommunications industry have modulated LiNbO3 in waveguides to generate blazed gratings. Kulishov et al. have reported “interdigitated electrode (IDE) structures” for use in dynamically-controlled, in-fiber, blazed gratings. They worked with an electrode pitch of 4 μm and a waveguide thickness of 1 μm, and Δn was found about 0.004. However, they did not require the maximum phase shift φmax of about 2π that would be required by a blazed grating, and therefore their technology is not directly applicable to beam steering.
Other researchers have sought to apply electro-optic crystals more directly to the beam-steering challenge, but have only succeeded to date in steering through very fine angles. Thomas et al. have developed a dynamic grating based on PLZT, in which a maximum phase shift φmax of about 2π for about 0.632 μm is achieved by applying a voltage of 318V. The grating period was 400 μm, the modulator thickness was 350 μm, and a steering angle of about ±0.05o was achieved. In order to achieve higher steering angles, a more optically-active material is required.